The present invention relates to chemical mechanical planarization (CMP), and more particularly, to optical endpoint detection during a CMP process, and specifically to prediction of that endpoint.
Chemical mechanical planarization (CMP) has emerged as a crucial semiconductor technology, particularly for devices with critical dimensions smaller than 0.5 micron. One important aspect of CMP is endpoint detection (EPD), i.e., determining during a polishing process when to terminate the polishing process.
Many users prefer EPD systems that are xe2x80x9cin situ EPD systemsxe2x80x9d, which provide EPD during the polishing process. Numerous in situ EPD methods have been proposed, but few have been successfully demonstrated in a manufacturing environment and even fewer have proved sufficiently robust for routine production use.
One group of prior art in situ EPD techniques involves the electrical measurement of changes in the capacitance, the impedance, or the conductivity of the wafer and calculating the endpoint based on an analysis of this data. To date, these particular electrically based approaches to EPD do not appear to be commercially viable.
Another electrical approach that has proved production worthy is to sense changes in the friction between the wafer being polished and the polish pad. Such measurements are done by sensing changes in the motor current. These systems use a global approach, i.e., the measured signal assesses the entire wafer surface. Thus, these systems do not obtain specific data about localized regions. Further, this method works best for EPD for metal CMP because of the dissimilar coefficient of friction between the polish pad and the layers of metal film stacks such as a tungsten-titanium nitride-titanium film stack versus the coefficient of friction between the polish pad and the dielectric underneath the metal. However, with advanced interconnection conductors, such as copper (Cu), the associated barrier metals, e.g., tantalum or tantalum nitride, may have a coefficient of friction that is similar to the underlying dielectric. The motor current approach relies on detecting the copper-tantalum nitride transition, then adding an overpolish time. Intrinsic process variation in the thickness and composition of the remaining film stack layer mean that the final endpoint trigger time may be less precise than is desirable.
Another group of methods uses an acoustic approach. In a first acoustic approach, an acoustic transducer generates an acoustic signal that propagates through the surface layer(s) of the wafer being polished. Some reflection occurs at the interface between the layers, and a sensor positioned to detect the reflected signals can be used to determine the thickness of the topmost layer as it is polished. In a second acoustic approach, an acoustical sensor is used to detect the acoustic signals generated during CMP. Such signals have spectral and amplitude content that evolves during the course of the polish cycle. However, to date there has been no commercially available in situ endpoint detection system using acoustic methods to determine endpoint.
Finally, the present invention falls within the group of optical EPD systems. An optical EPD system is disclosed in U.S. Pat. No. 5,433,651 to Lustig et al. in which light transmitted through a window in the platen of a rotating CMP tool and reflected back through the window to a detector is used to sense changes in a reflected optical signal. However, the window complicates the CMP process because it presents to the wafer an inhomogeneity in the polish pad. Such a region can also accumulate slurry and polish debris that can cause scratches and other defects.
Another approach is of the type disclosed in European application EP 0 824 995 A1, which uses a transparent window in the actual polish pad itself. A similar approach for rotational polishers is of the type disclosed in European application EP 0 738 561 A1, in which a pad with an optical window is used for EPD. In both of these approaches, various means for implementing a transparent window in a pad are discussed, but making measurements without a window was not considered. The methods and apparatuses disclosed in these patents require sensors to indicate the presences of a wafer in the field of view. Furthermore, integration times for data acquisition are constrained to the amount of time the window in the pad is under the wafer.
In another type of approach, the carrier is positioned on the edge of the platen so as to expose a portion of the wafer. A fiber optic based apparatus is used to direct light at the surface of the wafer, and spectral reflectance methods are used to analyze the signal. The drawback of this approach is that the process must be interrupted in order to position the wafer in such a way as to allow the optical signal to be gathered. In so doing, with the wafer positioned over the edge of the platen, the wafer is subjected to edge effects associated with the edge of the polish pad going across the wafer while the remaining portion of the wafer is completely exposed. An example of this type of approach is described in PCT application WO 98/05066.
In another approach, the wafer is lifted off of the pad a small amount, and a light beam is directed between the wafer and the slurry-coated pad. The light beam is incident at a small angle so that multiple reflections occur. The irregular topography on the wafer causes scattering, but if sufficient polishing is done prior to raising the carrier, then the wafer surface will be essentially flat and there will be very little scattering due to the topography on the wafer. An example of this type of approach is disclosed in U.S. Pat. No. 5,413,941. The difficulty with this type of approach is that the normal process cycle must be interrupted to make the measurement.
A further approach entails monitoring absorption of particular wavelengths in the infrared spectrum of a beam incident upon the backside of a wafer being polished so that the beam passes through the wafer from the nonpolished side of the wafer. Changes in the absorption within narrow, well defined spectral windows correspond to changing thickness of specific types of films. This approach has the disadvantage that, as multiple metal layers are added to the wafer, the sensitivity of the signal decreases rapidly. One example of this type of approach is disclosed in U.S. Pat. No. 5,643,046.
A method is provided for use with a tool for polishing thin films on a semiconductor wafer surface that predicts an endpoint of a polishing process. In one embodiment, the method utilizes an apparatus that includes a polish pad having a through-hole, which is in optical communication with a light source through a fiber optic cable assembly. The apparatus also includes a light sensor, and a computer. The light source provides light within a predetermined bandwidth. The fiber optic cable propagates the light through the through-hole to illuminate the wafer surface during the polishing process. The light sensor receives reflected light from the surface through the fiber optic cable and generates data corresponding to the spectrum of the reflected light. The computer receives the reflected spectral data (the xe2x80x9creflected signalxe2x80x9d) and generates a signal as a function of the reflected spectrum (the xe2x80x9creflectance spectrumxe2x80x9d, i.e., a gathered reflectance spectrum). The generated signal is then compared to spectra taken from other similar wafers (the xe2x80x9creference spectrumxe2x80x9d) processed prior to the current wafer. The comparison involves using any of many available methods to generate a difference between the reflected signal and the reference signal to provide data points that may, for ease of explanation, be graphically visualized as difference (y-axis) vs. time (x-axis). (The calculation may, of course, be done using other statistical analysis methods as well.) The computer then calculates a trigger time by calculating the slope between the graphed comparison data points, and then fitting a best-fit line to the data points, and extrapolating the best-fit line to cross the time axis resulting in a time intercept, which is the trigger time. Then, a preset constant value is added to the time intercept (trigger time) resulting in an endpoint time. At the endpoint time or at a given time established as a known completion time, if the endpoint time has not occurred, the polishing process is terminated.
Optical endpoint detection is accomplished by comparing a gathered reflectance spectrum to a reference spectrum. The reference spectrum is obtained by polishing a reference wafer to a process of record (POR) polish time and using the POR conditions while collecting the reflectance spectra at time intervals from the wafer. A reflectance spectrum from a selected time period just prior to the completion of polishing is then designated as the reference spectrum. One or more wafers may be used to establish the reference spectrum.
For wafers with a metal film to be polished, the reference signal and corresponding reference spectrum are typically selected at a time that corresponds to stable polishing of the metal film before the onset of clearing the metal film occurs. When clearing occurs, the reflected spectrum is substantially different from the reference spectrum taken during the metal phase. Since the metal film reflectance spectrum is similar from wafer to wafer, the reference spectrum may be taken from a reference wafer, or it may be taken each time a wafer is polished from the wafer itself, during the bulk metal polishing phase before any clearing takes place.
If it is desired to generate an endpoint on a barrier film between the metal film and a dielectric layer, the reference spectrum may be taken from the barrier layer of the appropriate reference wafer.
For dielectric film wafers, where the film reflectance changes during polishing, it is preferred to take a reference spectrum near the desired end point from a reference wafer. If it is desirable to know when, for example, half of the dielectric layer has been removed, a reference spectrum should be taken from the reference wafer that corresponds to half of the film being removed. The selection of the reference spectrum corresponds to the desired information from the film being polished.
Production wafers are then polished and the reflectance spectrum is continuously sampled at the selected time intervals. A comparison is made between the reference spectrum and the reflectance spectrum sometime before a point in time when the process would be known to be completed. Data generated from the comparison, if visualized as graphed over time, would indicate a convergence as the sampled signals gathered became closer in magnitude. A best-fit line is then determined for the endpoint signal data generated from the comparison, and the line is extrapolated to the x-axis to determine a trigger time. A predetermined amount of time is then added to the trigger time to produce an endpoint time. When the endpoint time is reached the polishing process ends. The polishing process may also end if a time predicted exceeds an acceptable value such as the total time required to polish the reference wafer.
This Summary of the Invention section is intended to introduce the reader to aspects of the invention and is not a complete description of the invention. Particular aspects of the invention are pointed out in other sections here below and the invention is set forth in the appended claims, which alone demarcate its scope.